Sprayed Concrete

Sprayed concrete, also known as shotcrete, is a construction technique where concrete or mortar is conveyed through a hose and pneumatically projected at high velocity onto a receiving surface. This technique can be offered by means of either a wet-mix or dry-mix process, with the selection of process, impacted by the project schedule, budget, access, available materials and numerous other factors.

Wet-mix Process

Wet-mix shotcrete pumps prepared concrete (typically ready-mixed concrete) into the nozzle. Compressed air is then introduced at the nozzle to impel the mixture onto a receiving surface. Compared to the dry-mix process, the wet process produces less rebound waste (when material falls to the floor), less dust and can place larger volumes in less time. The force used to eject shotcrete from the nozzle places and compacts/consolidates at the same time enabling it to be sprayed onto any type or shape of surface – including vertical or overhead areas.

Dry-mix Process

The dry-mix method involves placing the dry ingredients into a hopper and then conveying them pneumatically through a hose to the nozzle. The nozzle operator then controls the correct addition of water before the mixture hits the receiving surface. This requires a skilled nozzleman, especially in the case of thick or heavily reinforced sections. Advantages of the dry-mix process are that the water content can be adjusted instantaneously, allowing more effective placement in overhead and vertical applications without using accelerators. The dry-mix process is useful in repair applications when frequent stops are necessary and the dry material is easily discharged from the hose. Gunite was originally a trademarked name that specifically referred to the dry-mix pneumatic cement application process.